Silica by precipitation at constant alkali number, and its use

ABSTRACT

Precipitated silica or silicates, obtainable by acid precipitation of aqueous silicate solutions while maintaining a constant alkali number of at least 1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to silicas and silicates obtained and obtainableby acid precipitation of alkali metal silicates at constant alkalinumber, and to their use, for example as carriers.

2. Discussion of the Background

Precipitated silicas as carrier materials, particularly for vitamin Eacetate or choline chloride have long been known. For example, EP 0 937755 describes how a precipitated silica is prepared by a pH-controlledprecipitation reaction and then spray-dried. Precipitated silicaprepared in this way is particularly suitable for use for adsorbingliquid active substances such as choline chloride solution or vitamin E,for example. DE 198 60 441 discloses how an active substance adsorbatemay be prepared from a precipitated silica and an active substance byspraying or injecting a silica suspension together with one or moreactive substances into a fluidized bed generated using hot air. It islikewise possible to use hydrophobic precipitated silicas for thesepurposes, as described in DE 198 25 687.

In the context of their use as carriers, the following properties ofsilicas are important:

adsorption capacity, good sorption kinetics, and low fine dust fraction.Owing to heightened safety requirements and the need to prepareadsorbates with ever higher concentrations, there is therefore a demandfor carrier silicas which have a very low fines fraction with anadsorptiveness which is heightened at the same time.

OBJECTS OF THE INVENTION

The known silicas generally do not possess pronounced sorptioncharacteristics for polar compounds. Since silicas are frequently usedas carrier material for polar compounds such as choline chloride,propionic acid or formic acid, for example, it is one object of thepresent invention to provide a silica and a silicate which possessesparticularly good sorbency for polar compounds.

SUMMARY OF THE INVENTION

It has surprisingly been found that by preparing precipitated silica andsilicate at a constant alkali number, products can be obtained whichhave good sorption characteristics for polar compounds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides such precipitated silicas and silicatesas well as a process for preparing such materials comprising

-   -   optionally providing an initial aqueous silicate solution    -   simultaneously metering into an aqueous silicate solution or a        vessel, etc., an aqueous silicate solution and a Lewis and/or        Brønsted acid to provide a mixture    -   reacidifying the mixture to a pH of 7-3.0    -   optionally filtering, and    -   optionally drying,        the metered addition (or mixing, etc.) of the aqueous silicate        solution and the Lewis and/or Brønsted acid being carried out        while maintaining a constant alkali number of at least 1.

Where an aqueous silicate solution is initially present it is generallydifferent from the silicate solution metered in as concentrationsgenerally differ due to the initial presence of water. See infra.

While not bound by theory it is believed that the high alkali number ofthe silicas and silicates of the invention, and the inventively preparedprecipitated silicas and silicates, results in a high silanol groupdensity and thus enhances a high absorbency for polar absorbates.

The present invention likewise provides for the use of the silicas andsilicates of the invention as carrier material, for example for feedadditives, chemical intermediates, or in the laundry detergent industry,for example.

It is possible to add an electrolyte prior to or during the simultaneousaddition (or metering) of aqueous silicate solution and acid.Electrolytes for the purposes of the present invention are not limitedand include metal salts or their aqueous solutions which are notincorporated into the amorphous SiO₂ structure, such as Na, K, Rb, Ba,in each case in sulfate, acetate, halide or carbonate form. The fractionof the electrolyte is 0.01-26% by weight (calculated as the metal ion)based on total weight of product silica or silicate.

It is likewise possible to add metal salts or their solutions which areincorporated into the SiO₂ structure to the precipitation mixture, sogiving silicates. The fraction of these metal ions may be between 1 and50, preferably 10% by weight based on total weight of silicate;customary ions are Al, Zr, Ti, Fe, Ca and Mg.

There are known preparation processes for precipitated silicas in thecourse of which a constant pH is maintained. A precipitation reaction atconstant alkali number, on the other hand, means that the concentrationof freely available alkali ions (e.g., sodium ions) is kept constant.

As a result of the acid-base reactions during the precipitation ofwaterglass with sulfuric acid, sodium ions are on the one hand releasedin the form of sodium sulfate; on the other hand, sodium ions areincorporated into the silicate agglomerates which form.

Since these two reactions proceed independently of one anotherkinetically, the course of precipitations at constant pH is differentthan that of precipitations conducted in accordance with the invention.

In the case of a precipitation reaction at constant alkali number, thepH changes analogously: for example, at a constant alkali number of 30,the pH falls from about 10.35 to levels between 8 and 10, depending onthe duration of the precipitation reaction (simultaneous addition ofalkaline silicate solution and acid). The longer such a precipitationreaction lasts, the lower the pH at the end of the reaction is. Theintercalation of sodium ions into the silica structure is probablyresponsible for this.

The precipitated silicas and silicates of the invention are prepared atan alkali number of at least 1, in particular at least 15, preferably atan alkali number of from 15 to 60, with particular preference at analkali number of from 25 to 50, and with very particular preference atan alkali number of from 30 to 40, including all of 2, 5, 10, 15, 20,30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 45 and 55, the alkali numberbeing kept constant during the precipitation reaction.

The alkali number (AN) is determined by measuring the consumption ofhydrochloric acid in a direct potentiometric titration of theprecipitation suspension at a pH of 8.3, i.e., the color change point ofphenolphthalein. The consumption of hydrochloric acid is a measure ofthe free alkali content of the solution or suspension. Owing to thetemperature dependency of the pH, this measurement is made at 40° C. andafter a waiting time of 15 minutes. A precise description of themeasurement protocol is given in the examples.

The invention provides a precipitated silica, characterized by thefollowing physicochemical data:

-   BET surface area from 50 to 700 m²/g with the preferential ranges    100-300 m²/g, 150-220 m²/g, 180-210 m²/g,-   DBP absorption from 100 to 450 g/100 g with the preferential ranges    250-450 g/100 g, 280-450 g/100 g,-   Choline chloride from 50 to 400 g/100 g with the absorption    preferential ranges 240-400 g /100 g, 280-400 g/100 g (75% by weight    aqueous solution),-   CTAB surface area from 50 to 350 m²/g with the preferential ranges    100-250 m²/g, 130-200 m²/g,    the ratio of the DBP absorption to the choline chloride absorption,    as a measure of the adsorption of a nonpolar substance and of a    polar substance, being less than 1.07, preferably less than 1.05,    with very particular preference less than 1.03.

Since silicas or silicates possess different affinities for hydrophobic,i.e., nonpolar, and hydrophilic, i.e., polar, compounds, twomeasurements are necessary for complete characterization of thisproperty. The DBP number is used as a measure of an affinity of silicasfor hydrophobic compounds; the choline chloride absorption is used as ameasure of the affinity of silicas for hydrophilic compounds. The ratioof these DBP/choline chloride absorption figures therefore reflects anew physical property.

The silicates or precipitated silicas of the invention may additionallybe characterized by the modified Sears number. The modified Sears numberis determined by the techniques described in the examples/methods andmay be greater than 20, preferably greater than 25, with particularpreference greater than 28. Preferably the modified Sears number is 45or less.

The preferred aqueous silicate solution is sodium silicate solution; asthe Brønsted acid, sulfuric acid, hydrochloric acid, carbonic acid oracetic acid may be used. As the Lewis acid it is possible to use Al³⁺ions, in the form for example of the sulfate.

The BET surface area is determined in accordance with ISO 5794/1, AnnexD, incorporated herein by reference, the CTAB surface area in accordancewith ASTM D 3765-92, incorporated herein by reference, the DBPabsorption in accordance with the protocol described in the annex.

The precipitated silica suspensions prepared by the process of theinvention may be filtered conventionally and the filter cake may bewashed with water. The filter cake obtained in this way is liquefied,where appropriate, and may be dried by the customary drying process,such as rotary tube furnace, Büttner dryer, spin-flash dryer, pulsecombustion dryer, spray dryer, or in a nozzle tower. Further, purelyphysical treatment by granulation and/or grinding is likewise possible.Also possible is a water repellency treatment or coating with waxes.

The silicates or precipitated silicas of the invention may be used inparticular as carriers, for example for feed additives such as formicacid, propionic acid, lactic acid, phosphoric acid, choline chloridesolution or plant extracts, tagatese extract for example.

Furthermore, the precipitated silicas of the invention may be used ascarrier material for chemical intermediates such as melamine resins orcoatings additives or in the laundry detergent industry as carriers forfragrances or detergents.

Moreover, the silicates or precipitated silicas of the invention may beused as a filler in elastomers/plastics, battery separators,toothpastes, catalyst supports, or as a flocculation assistant.

The following examples and measurement protocols are intended toillustrate the invention without restricting its scope.

EXAMPLES General Experimental Protocol Example 1-8

Water is charged to a precipitation vessel with a capacity of 2 m³(applies to all pilot-scale trials; laboratory trials: 40 l; planttrials: 80 m³) and a certain amount of waterglass (i.e., sodium silicatesolution) is metered in. The values for the density of the sodiumsilicate solution, sulfuric acid, the SiO₂ content, Na₂O content,temperature, and the alkali number (AN number) are evident from thetables. After the target temperature has been reached, sodium silicatesolution and sulfuric acid are metered in. Thereafter, sulfuric acidcontinues to be metered in at the same rate until a pH of 3.5 isreached. The suspension with the described solids content is filtered onfilter presses (membrane filter presses) and then prepared for drying.The filter cake is liquefied by adding sulfuric acid, using a shearingunit, until the desired viscosity and pH are reached. The feed is thendried.

List of abbreviations:

-   AN=alkali number-   WGL charge=waterglass charge=initial charge of sodium silicate    solution-   WGL=waterglass-   VA=time at which the viscosity increases sharply, also referred to    as the gel point

Fc  =  precipitation  rate  in  [mol/(1  ·  min)],$\text{defined~~by~~}\frac{\text{ml/min~~(acid~~inflow)~~·~~mol/l~~(acid~~molarity)}}{\text{l~~(initial~~charge)}}$

-   % TS feed=% solids content of feed to dryer-   GV-Din=loss on ignition to DIN-   LF=conductivity-   CC absorption=choline chloride absorption

Example 1 2 3 4 5 6 Trial No. 7508 7504 7487 7491 7397 7362 AN 30 40 4040 20 30 Water charge (l) 1359 1220 1314 1220 1651 1499 WGL charge (l)145.3 186.1 197.6 184.1 108.4 148.8 Temperature (° C.) 85 85 85 85 85 85WGL metered (l) 344.2 211.4 247.9 249.2 345.0 441.8 Acid metered (l)26.32 16.42 19.32 20.15 28.40 36.00 VA (min) 22.50 37.00 38.50 41.0024.75 27.75 Precipitation period (min) 60 55 55 65 40 65 Reacidificationamount (l) 20.00 22.30 25.11 22.86 20.42 28.41 Reacidification time(min) 21 39 156 41 63 Fc 5.52 4.01 4.39 4.17 7.21 5.96 g/l (solidscontent of the earth suspension) 94.8 92.5 97.7 99.2 79.3 104.0Waterglass analysis Density (g/ml) 1.346 1.346 1.349 1.349 1.346 1.348 %SiO₂ 27.2 27.2 27.3 27.3 27.4 27.2 % Na₂O 7.99 7.99 8.08 8.08 8.03 8.02Sulfuric acid (mol/l H₂SO₄) 18.14 18.14 18.14 18.14 18.87 17.74 DryerNozzle tower Nozzle tower Nozzle tower Nozzle tower Nozzle tower Nozzletower dryer dryer dryer dryer dryer dryer pH of feed 3.9 3.8 3.8 3.8 3.63.6 Viscosity (mPa*s) 40 75 110 120 75 60 % TS feed 17.3 18.3 19.7 20.316.3 16.9 Analysis GV-DIN (%) 5.2 4.9 4.9 4.9 5.1 5.3 Water (%) 6.2 5.36.3 6.0 5.8 6.2 pH 6.5 6.5 6.9 6.1 6.8 6.7 LF (μS/cm) 610 600 550 650490 510 N₂ surface area (m²/g) 199 189 167 165 240 175 CTAB surface area(m²/g) 148 127 120 113 200 135 DBP number (g/100 g) (corrected, i.e.,for 279 281 270 270 247 293 anhydrous material) Sears number (V2) 31.330.9 29.4 29.5 31.0 31.5 CC absorption 276 282 270 275 233 285 Ratio ofDBP/CC absorption 1.0109 0.9965 1.0000 0.9818 1.0601 1.0281 HubersilHiSil Sipernat 5170 SC72 Sipernat 22 2200 Example 7 8 9 10 11 12 TrialNo. 7370 7420 AN 30 40 Water charge (l) 1459 1224 WGL charge (l) 148.8185.3 Temperature (° C.) 85 85 WGL metered (l) 542.5 224.4 Acid metered(l) 32.78 18.44 VA (min) 27.25 40.25 Precipitation period (min) 60 60Reacidification amount (l) 24.70 26.37 Reacidification time (min) 56 94Fc 6.03 3.86 g/l (solids content of the earth suspension) 100.0 92.5Waterglass analysis Density (g/ml) 1.348 1.351 % SiO₂ 27.2 27.6 % Na₂O8.02 8.01 Sulfuric acid (mol/l H₂SO₄) 17.64 17.70 Dryer Nozzle towerNozzle tower Granulation Nozzle tower Spray dryer Nozzle tower dryerdryer dryer dryer pH of feed 3.4 3.6 Viscosity (mPa*s) 33 90 % TS feed15.3 19.8 Analysis GV-DIN (%) 5.8 5.6 Water (%) 5.0 5.9 6.0 5.7 5.0 5.0pH 6.3 6.6 LF (μS/cm) 530 550 N₂ surface area (m²/g) 185 137 CTABsurface area (m²/g) 148 115 DBP number (g/100 g) (corrected, i.e., for292 276 204 311 270 255 anhydrous material) Sears number (V2) 30.8 28.518.4 23.5 CC absorption 294 279 165 270 235 231 Ratio of DBP/CCabsorption 0.9932 0.9892 1.2364 1.1519 1.149 1.1039Determining the Modified Sears Number of Silicas, Silicates andHydrophobic Silicas1. Scope

Free OH groups are detectable by titration with 0.1 N KOH in the rangefrom pH 6 to pH 9.

2. Apparatus

-   -   2.1 Precision balance to 0.01 g precisely    -   2.2 Memotitrator DL 70, Mettler, equipped with 10 ml and 20 ml        buret, 1 pH electrode and 1 pump (e.g., NOUVAG pump, type SP        40/6)    -   2.3 Printer    -   2.4 Titration vessel 250 ml, Mettler    -   2.5 Ultra-Turrax 8000-24,000 rpm    -   2.6 Thermostated waterbath    -   2.7 2 dispenser 10-100 ml for metering methanol and deionized        water    -   2.8 1 dispenser 10-50 ml for metering deionized water    -   2.9 1 measuring cylinder 100 ml    -   2.10 IKA universal mill M20        3. Reagents    -   3.1 Methanol p.a.    -   3.2 Sodium chloride solution (250 g NaCl p.a. in 1000 ml        deionized water)    -   3.3 0.1 N hydrochloric acid    -   3.4 0.1 N potassium hydroxide solution    -   3.5 Deionized water    -   3.6 Buffer solutions pH 7 and pH 9        4. Procedure    -   4.1 Sample Preparation

Grind about 10 g of sample for 60 seconds in the IKA universal mill M20.

Important: Since only very finely ground samples lead to reproducibleresults, these conditions must be observed strictly.

-   -   4.2 Analytical Procedure        -   4.2.1 Weigh out 2.50 g of the sample prepared in accordance            with section        -   4.1 into a 250 ml titration vessel.        -   4.2.2 Add 60 ml of methanol p.a.        -   4.2.3 After complete wetting of the sample, add 40 ml of            deionized water.        -   4.2.4 Disperse for 30 seconds using the Ultra-Turrax at a            speed of about 18,000 rpm.        -   4.2.5 Rinse sample particles adhering to the vessel edge and            stirrer into the suspension using 100 ml of deionized water.        -   4.2.6 Condition sample to 25° C. in a thermostated water            bath (for at least 20 minutes).        -   4.2.7 Calibrate pH electrode with the buffer solutions pH 7            and pH 9.        -   4.2.8 The sample is titrated in the Memotitrator DL 70 in            accordance with method S 911. If the course of titration is            unclear, a duplicate determination is carried out            subsequently.            The results printed out are as follows:

-   pH

-   V₁ in ml/5 g

-   V₂ in ml/5 g    Principle:

First of all the initial pH of the suspension is measured, thenaccording to the result the pH is adjusted to 6 using KOH or HCl. Then20 ml of NaCl solution are metered in. The titration is then continuedto a pH of 9 using 0.1 N KOH.

Sears numbers:Si—OH+NaCl

Si—ONa+HClHCl+KOH

KCl+H₂OCalculation

$V_{1} = \frac{V*5}{E}$

$V_{2} = \frac{V*5}{E}$

-   V₁=ml KOH or ml HCl to pH 6/5 g of substance-   V₂=ml KOH consumption to pH 9/5 g of substance-   E=initial mass

Titraton was conducted on a Memotitrator DL 70 with a switch-off delaytime of 2s.

Determining the Alkali Number:

The alkali number determination, referred to below for short as ANdetermination, is the consumption of hydrochloric acid in a directpotentiometric titration of alkaline charges or suspensions to a pH of8.3 (viewed historically: pH 8.3 corresponds to the color change pointof phenolphthalein); this gives the free alkali content of the solutionor suspension.

The pH meter is calibrated at room temperature, the combined electrodeis equilibrated to 40° C., and the sample mixture is then conditioned at40° C., and on reaching that temperature the titration is conducted.

Because of the fairly long time for equilibrium to be establishedbetween the silica/silicate at the specified pH—in this case 8.3—awaiting time is necessary until the consumption of acid is finally readoff. Extensive investigations have found that for the AN determination awaiting time of 15 minutes must be observed, after which the equilibriumhas established itself stably and good reproducibility is ensured.

Description of Method:

pH Meter Calibration:

-   -   Calibrating temperature of buffer solutions 20° C.    -   Temperature equilibration 20° C.        Measurement of Suspension:    -   Temperature equilibration of pH meter at 40° C.    -   50 ml of suspension    -   50 ml of distilled water    -   Hydrochloric acid c=0.5 mol/l    -   Condition suspension to 40° C.    -   Determine acid consumption after 15 min titration time    -   Titration end at pH 8.3        Accuracy of method: +/−0.1 ml acid consumption        Determining the Maximum Choline Chloride Absorption:        Test Means:        A. Test Apparatus:

-   250 ml glass beaker, high form

-   Spatula

-   Precision balance    B. Test substances:

-   75% strength choline chloride solution [choline chloride, ultrapure    (Merck)]

-   Silica under test    Calibration Notes

When a new delivery test solution is received, it must be examinedcomparatively with the quality used up until that time.

Before use, the balances are to be tested for functionality and servicedannually.

Procedure:

10 g of the carrier silica under test are weighed out into a 250 mlglass beaker, high form, and 75% strength choline chloride solution isadded dropwise, while stirring with the spatula. The mixture is observedcontinuously to check when the maximum absorption has been reached. Whenviewed closely, it is possible to make out white silica particles whichstand out distinctly from waxlike (saturated) particles. The maximumcholine chloride absorption has been achieved when there are no longerany unladen particles in the mixture and this mixture is not yetwaxlike/greasy.

Evaluation:

${{{Max}.\;{choline}}\mspace{14mu}{chloride}\mspace{14mu}{absorption}\mspace{14mu}{in}\mspace{14mu}{g/100}\mspace{14mu} g} = \frac{\left( {a - 10} \right) \times 100}{10}$

-   a=total weight

German application 101 12 441.4 filed Mar. 15, 2001 is incorporatedherein by reference.

In the invention process, the “metered addition” of aqueous silicatesolution and acid may be addition to a pre-existing aqueous silicatesolution, or simply to, e.g., an empty vessel (e.g., controlled mixing).The rate of addition of components is guided by the control of thealkali number, and is within the skill of the ordinary artisan in viewof the disclosure above.

1. A silica having the following physicochemical characteristics: BETsurface area from 50 to 700 m²/g; DBP absorption from 100 to 450 g/100g; Choline chloride from 150 to 400 g/100 g (75% ab- absorption sorptionby weight aqueous solution); CTAB surface area from 50 to 350 m²/g;DPB/choline chloride absorption less than 1.07; and Sears number greaterthan 25 ml/5 g.


2. The silica of claim 1, having a modified Sears number of from atleast 25 to
 45. 3. The silica of claim 1, having a BET surface area of180-210 m²/g, a DBP adsorption of 280-450 g/100 g, and a CTAB surfacearea of 130-200 m²/g.
 4. A process for preparing precipitated silica,comprising: simultaneously metering into an aqueous silicate solutionmore aqueous silicate solution and a Lewis and/or Brønsted acid over aprecipitation period of 40 to 65 minutes followed by reacidifying themixture to a pH of 7-3.0 to provide an acidified mixture having a solidcontent of the suspension of from 79.3 to 104 g/l, optionally filteringthe acidified mixture to obtain a filtered precipitated silica,optionally drying the filtered precipitated silica, wherein the meteredaddition of the aqueous silicate solution and the Lewis and/or Brønstedacid is carried out while maintaining a constant alkali number in themixture of at least 15, and wherein said silica has the followingphysicochemical characteristics: BET surface area from 50 to 700 m²/g;DBP absorption from 100 to 450 g/100 g; Choline chloride from 150 to 400g/100 g (75% ab- absorption sorption by weight aqueous solution); CTABsurface area from 50 to 350 m²/g; DPB/choline chloride absorption lessthan 1.07; and Sears number greater than 25 ml/5 g.


5. The process of claim 4, wherein the alkali number is of from 25 to50.
 6. The process of claim 4, further comprising the addition of anelectrolyte prior to or during the simultaneous addition of aqueoussilicate solution and Lewis and/or Brønsted acid.
 7. A method comprisingcontacting the precipitated silica of claim 1 with a feed additive, achemical intermediate, or a laundry detergent component.
 8. A methodcomprising contacting the precipitated silica of claim 1 with formicacid, propionic acid, lactic acid, phosphoric acid, choline chloridesolution, a plant extract, a melamine resin, a coatings additive, afragrance, or a detergent.
 9. An elastomer, plastic, battery separator,toothpaste, catalyst support or flocculation assistant comprising theprecipitated silica of claim
 1. 10. A process for preparing precipitatedsilica, comprising: simultaneously metering into a vessel an aqueoussilicate solution and a Lewis and/or Brønsted acid over a precipitationperiod of 40 to 65 minutes followed by reacidifying the mixture to a pHof 7-3.0 to provide an acidified mixture having a solid content of thesuspension of from 79.3 to 104 g/l, optionally filtering the acidifiedmixture to obtain a filtered precipitated silica, optionally drying thefiltered precipitated silica, wherein the metered addition of theaqueous silicate solution and the Lewis and/or Brønsted acid is carriedout while maintaining a constant alkali number in the mixture of atleast 15, and wherein said silica has the following physicochemicalcharacteristics: BET surface area from 50 to 700 m²/g; DBP absorptionfrom 100 to 450 g/100 g; Choline chloride from 150 to 400 g/100 g (75%ab- absorption sorption by weight aqueous solution); CTAB surface areafrom 50 to 350 m²/g; DPB/choline chloride absorption less than 1.07; andSears number greater than 25 ml/5 g.


11. The process of claim 10, wherein the alkali number is of from 25 to50.
 12. The process of claim 10, further comprising the addition of anelectrolyte prior to or during the simultaneous addition of aqueoussilicate solution and Lewis and/or Brønsted acid.
 13. The process ofclaim 4, wherein said filtering of said acidified mixture is performed.14. The process of claim 13, wherein said drying of said filteredprecipitated silica is performed.
 15. The process of claim 10, whereinsaid filtering of said acidified mixture is performed.
 16. The processof claim 15, wherein said drying of said filtered precipitated silica isperformed.
 17. The precipitated silica of claim 1, having a modifiedSears number greater than 28 ml/5 g.
 18. The precipitated silica ofclaim 1, having a modified Sears number greater than 28 ml/5 g.
 19. Theprecipitated silica of claim 1, having a modified Sears number greaterthan 28 ml/5 g.